Sunday, January 23, 2011

Cultivars of the American Elm

'Princeton' elms near the White House.

Why do we plant different cultivars of the American Elm? Surely there's nothing quite like the original Ulmus americana. The classic American Elm is hardy, fast-growing and can grow into a healthy and aesthetically beautiful specimen with ease. However, the prevalence of Dutch elm disease (DED) since the middle of the 20th century has threatened the future of the elms that once lined streets and parks throughout the District and populated America's main streets. Certain cultivars have been developed to be resistant to DED, or have been discovered to be resistant to the disease.

Certain varieties, such as Washington and Jefferson are classic American elms that were planted by the National Park Service in Washington, and can be found on the National Mall and the monumental core of the city. These were naturally occurring varieties but are now reproduced as clones to ensure the identical genetic makeup.

The 'Princeton' cultivar is a variety developed in a nursery; it must be reproduced as a clone or it will revert to one of its parents. 'Princeton,' which can be seen planted in rows in front of the White House along Pennsylvania Avenue, was developed in the 1920s before the threat of Dutch elm disease. It has come into wider use because it has shown good resistance to DED. 'Valley Forge' is a more recent U.S. National Arboretum cultivar and is probably the most resistant to DED. ‘Princeton’ and ‘Valley Forge’ are the primary cultivars Casey Trees has planted during the past seven years. Casey Trees is planting 100 'Valley Forge' elms this winter; in past years, we planted mostly 'Princetons.'

Cultivars vary not just in their disease and pest susceptibility, but also in their form. A ‘Princeton,’ for example, has a dense, symmetrical form with highly acute angles, making for an upright profile. It is notably vase-shaped, a good way you can identify it. The ‘Valley Forge’ also has a vase-shape, but the branching is looser.

There is a good reason why American Elms are cultivated carefully from single specimens rather than hybridized, as is done with many trees to select for certain traits. Hybridization within the genus Ulmus (crossing with other elm species) has been aimed primarily at breeding for DED and phloem necrosis resistance. However, most of the breeding and selection work does not include American elm because it is a tetraploid — that is, it has a chromosome number twice that of all other elms (56 versus 28). Therefore, most hybrid breeding and selection work does not include American elms. Thousands of attempts to cross the American with the Siberian elm have failed. Reports of successful artificial hybridization and verification of hybridizing American elm with other elms are rare.

Facts about American Elm cultivars:

Cultivars are crosses between clones. They are derived from root cuttings (rather than planted from seeds) to ensure their DED-resistance.

Although cultivars may be selected for DED-resistance, they may still be at risk for other elm diseases, such as elm yellows and verticillium wilt.

DED-resistance does not mean a cultivar is immune, but certain ones have significantly improved survival rates compared to other elms.